SMART provides a FHIR client (I believe in Javascript, Python and iOS).
HAPI-FHIR is Java a library that allows you to create a FHIR server and also FHIR clients.
A FHIR server exposes a RESTful API that clients can access.
You can certainly use the HAPI library to create a FHIR server, and use a SMART client to access that server, but there is not direct connection between them, just an agreed upon API.